The present invention relates, in general, to time interval measurement apparatus and methods.
Precise digital time interval measurement is an important part of the operation for many electronic sensor or conversion devices. The traditional method for conversion of a time to a numeric value (i.e. digital) is based on counting pulses from a constant frequency clock source.
If time interval to be measured is from time t1 to time t2, see FIG. 1, then the total duration t=t2−t1. The clock starts counting at time T1 and stops at the time T2. The interval time T is calculated by multiplying the time of the clock period Tclock by the number of counts N:T=Tclock·N
There is a time measurement error associated with this method that is due to the occurrence of the starting and stopping signals for the interval relative to the clock edges used for counting. This includes (T1−t1) and (T2−t2) and these differences can add up to be Tclock. This error can be reduced by decreasing Tclock (i.e., increasing the clock frequency: Fclock=1/Tclock). However, as the frequency increases, so does the complexity, power consumption and cost of the measurement circuit.
In sensors that determine the value of the measured variable by means of time interval measurement, very precise time measurement is a critical aspect of the precision of the sensor. In the past, high frequency counters (greater than 100 MHz) and Application Specific Integrated Circuits (ASICs) were used to achieve these very fine measurements of time. These circuits have inherent drawbacks which include high cost, high power consumption (i.e. not conducive to battery powered devices), and are prone to EMC noise emissions.
It would be desirable to provide a time measurement apparatus and method which addresses the deficiencies of prior time interval measurement apparatus and methods. It would also be desirable to provide a time interval measurement apparatus and method which can very precisely measure time intervals at high frequency rates.
It would also be desirable to provide a time interval measurement apparatus and method which has minimal measurement error without requiring increased clock frequencies. It would also be desirable to provide a time interval measurement apparatus and method which is not only capable of measuring time periods with extremely high resolution, but provides the measurement over a very long time period without jeopardizing the time resolution. It would also be desirable to provide a time interval measurement apparatus and method which has these features without requiring costly ASICs or high frequency oscillator and counter circuitry.